Pluto’s Moon Nix

Over the course of the past decade, many amazing discoveries have been made at the edge of the Solar System. Thanks to the work of astronomers working out of Earth-based observatories, with the Hubble Space Telescope, and those behind the recent New Horizons mission, not only have new objects been discovered, but additional discoveries have been made about the ones we already knew about.

For example, in 2005, two additional satellites were discovered in orbit of PlutoHydra and Nix. The discovery of these moons (which has since been followed by the discovery of two more) has taught astronomers much about the far-flung system of Pluto, and helped to advance our understanding of the Kuiper Belt.

Discovery and Naming:
Nix was discovered in June of 2005 by the Hubble Space Telescope Pluto Companion Search Team, using discovery images that were taken on May 15th and 18th, 2005. The team was composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young.

The discovery images of Nix (and Hydra) obtained by the Hubble Space Telescope. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI)
The discovery images of Nix (and Hydra) obtained by the Hubble Space Telescope. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI)

Nix and Hydra were also independently discovered by Max J. Mutchler on June 15th, 2005, and by Andrew J. Steffl on August 15th, 2005. At the time, Nix was given the provisional designation of S/2005 P 2 and casually referred to as “P2”. Once pre-recovery images from 2002 were confirmed, the discoveries were announced on October 31st, 2005.

In accordance with IAU guidelines concerning the naming of satellites in the Solar System, the moon was named Nix. Derived from Greek mythology, Nix is the goddess of darkness and night, the mother of Charon and the ferryman of Hades (the Greek equivalent of Pluto) who brought the souls of the dead to the underworld.

The name was officially announced on June 21st, 2006, in an IAU Circular, where the designation “Pluto II” is also given. The initials N and H (for Nix and Hydra) were also a deliberate reference to the New Horizons mission, which would be conducting a flyby of the Pluto system in less than ten years time after the announcement was made.

Images acquired by the New Horizon's probe of Nix (left) and Hydra (right). Credit: NASA/JHUAPL/SWRI
Images acquired by the New Horizon’s probe of Nix (left) and Hydra (right) on July 14th, 2015. Credit: NASA/JHUAPL/SWRI

Size, Mass and Orbit:
Based on observations with the Hubble Space Telescope of Nix’s geometric albedo and shape, the satellite was estimated to measure 56.3 km (35 mi) along its longest axis and 25.7 km (16 mi) wide. However, images provided by the New Horizons’ Ralph instrument on July 14th, 2015, indicated that Nix measures 42 km (26 mi) in length and 36 km (22 mi) wide.

Nix follows a circular orbit with very little eccentricity (0.0020) and a low inclination of approximately 0.13°. It is in the same orbital plane as Charon, is in a 3:2 orbital resonance with Hydra, and a 9:11 resonance with Styx. Its orbital period is roughly 24.9 days, meaning it takes about 25 days to complete a single orbit of Pluto.

As with Hydra and perhaps the other small Plutonian moons, Nix rotates chaotically, which is due mainly to its oblong shape. This means that the moon’s axial tilt and day length vary greatly over short timescales, to the point that it regularly flips over.

Early observations conducted by Marc W. Buie and William M. Grundy at the Lowell Observatory appeared to show that Nix has a reddish color like Pluto, but unlike any of its other moons. However, more-recent studies conducted by S. Alan Stern et al. using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS), have indicated that it is likely as grey as the remaining satellites.

From these observations, it is likely that the surface of Nix is composed primarily of water ice (like Hydra) and may or may also have trace amount of methane ice on its surface. If true, then the exposure of these deposits of methane ice to ultra-violet radiation from the Sun would result in the presence of tholins, which would give it a reddish hue.

However, when the New Horizons space probe photographed Hydra and Nix during its flyby of the Pluto system, it spotted a large region with a distinctive red tint, probably a crater. The appearance of this surface region – a spot of red against an otherwise grey landscape – may explain these conflicting results.

Thus far, only one mission has been performed to the Pluto system that resulted in close-up and detailed photographs of Nix. This would be the New Horizons mission, which flew through the Pluto-Charon system on July 14th, 2015 and photographed Hydra and Nix from an approximate distance of 640,000 km (400,000 mi).

Until July 13th, 2015, when NASA’s Long Range Reconnaissance Imager (LORRI) on board New Horizons determined Nix’s dimensions, its size was unknown. More images and information will be downloaded from the spacecraft between now and late 2016.

Prior to the discovery of Hydra and Nix in 2005, Pluto was believed to share its orbit with only the satellite of Charon – hence why astronomers often refer to it as the “Pluto-Charon system”. However, since the discovery of these two additional satellites in 2005, two more have been discovered – Kerberos in July of 2011 and Styx in July of 2012.

This raises the number of bodes in the Pluto-Charon system to one primary and five satellites. And thanks to the recent New Horizons flyby, we got to see all of them up close for the first time!

Like most large bodies in the Kuiper Belt (not to mention their satellites) much remains to be learned about Nix and its companions. In time, and with more missions to the outer Solar System, we are sure to address many of the mysteries surrounding this particular satellite, and will probably find many more waiting for us!

We have written many interesting articles on Pluto, its system of moons and the Kuiper Belt here at Universe Today.

Here’s Moons of Pluto, Pluto’s New Moons are Named Nix and Hydra, and Pluto’s Moons Nix and Hydra Get Real.

And here’s New Horizons Now Close Enough to See Pluto’s Smaller Moons, and Fifth Moon Found Around Pluto.

Astronomy Cast has a wonderful episode on the New Horizons mission, titled On Pluto’s Doorstep – Live Hangout with New Horizons Team.

For more information, check out NASA’s Solar System Exploration: Nix and’s page on updated images of Nix and Pluto’s other moons..

The Dwarf Planet Orcus

Since the early 2000s, more and more objects have been discovered in the outer Solar System that resemble planets. However, until they are officially classified, the terms Kuiper Belt Object (KBO) and Trans-Neptunian Object (TNO) are commonly used. This is certainly true of Orcus, another large object that was spotted in Pluto’s neighborhood about a decade ago.

Although similar in size and orbital characteristics to Pluto, Orcus is Pluto’s opposite in many ways. For this reason, Orcus is often referred to as the “anti-Pluto”, a fact that contributed greatly to the selection of its name. Although Orcus has not yet been officially categorized as a dwarf planet by the IAU, many astronomers agree that it meets all the requirements and will be in the future.

Discovery and Naming:
Orcus was discovered on February 17th, 2004, by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Although discovered using images that were taken in 2004, prerecovery images of Orcus have been identified going back as far as November 8th, 1951.

Provisionally known as 90482 2004 DW, by November 22nd, 2004, the name Orcus was assigned. In accordance with the IAU’s astronomical conventions, objects with a similar size and orbit to that of Pluto are to be named after underworld deities. Therefore, the discovery team suggested the name Orcus, after the Etruscan god of the underworld and the equivalent of the Roman god Pluto.

90482 Orcus. The location of Orcus is shown in the green circle (top, left). Credit: NASA
90482 Orcus. The location of Orcus is shown in the green circle (top, left). Credit: NASA

Size, Mass and Orbit:
Given its distance, estimates of Orcus’ diameter and mass have varied over time. In 2008, observations made using the Spitzer Space Telescope in the far infrared placed its diameter at 958.4 ± 22.9 km. Subsequent observations made in 2013 using the Herschel Space Telescope at submillimeter wavelengths led to similar estimates being made.

In addition, Orcus appears to have an albedo of about 21% to 25%, which may be typical of trans-Neptunian objects approaching the 1000 km diameter range. However, these estimates were based on the assumption that Orcus was a singular object and not part of a system. The discovery of the relatively large satellite Vanth (see below) in 2007 by Brown et al. is likely to change these considerably.

The absolute magnitude of Vanth is estimated to be 4.88, which means that it is about 11 times fainter than Orcus itself. If the albedos of both bodies are the same at 0.23, then the diameter of Orcus would be closer to 892 -942 km, while Vanth would measure about 260 -293 km.

In terms of mass, the Orcus system is estimated to be 6.32 ± 0.05 ×1020 kg, which is about 3.8% the mass of the dwarf planet Eris. How this mass is partitioned between Orcus and Vanth depends of their relative sizes. If Vanth is 1/3rd the diameter Orcus, its mass is likely to be only 3% of the system. However, if it’s diameter is about half that of Orcus, then its mass could be as high as 1/12 of the system, or about 8% of the mass of Orcus.

Orcus compared to Earth and the Moon. Credit: Wikipedia Commons
Orcus compared to Earth and the Moon. Credit: Wikipedia Commons

Much like Pluto, Orcus has a very long orbital period, taking 245.18 years (89552 days) to complete a single rotation around the Sun. It also is in a 2:3 orbital resonance with Neptune and is above the ecliptic during perihelion. In addition, it’s orbit has a similar inclination and eccentricity as Pluto’s – 20.573° to the ecliptic, and 0.227, respectively.

In short, Orcus orbits the Sun at a distance of 30.27 AU (4.53 billion km) at perihelion and 48.07 AU (7.19 billion km) at aphelion. However, Pluto and Orcus are oriented differently. For one, Orcus is at aphelion when Pluto is at perihelion (and vice versa), and the aphelion of Orcus’s orbit points in nearly the opposite direction from Pluto’s. Hence why Orcus is often referred to as the “anti-Pluto”.

The density of the primary (and secondary assuming they have the same density) is estimated to be 1.5 g/cm3. In addition, spectroscopic and near-infrared observations have indicated that the surface is neutral in color and shows signs of water. Further infrared observations in 2004 by the European Southern Observatory and the Gemini Observatory indicated the possible presence of water ice and carbonaceous compounds.

This would indicate that Orcus is most likely differentiated between a rocky core and an icy mantle composed of water and methane ices as well as tholins – though not as much as other KBOs which are more reddish in appearance. The water and methane ices are believed to cover no more than 50% and 30% of the surface, respectively – which would mean the proportion of ice on the surface is less than on Charon, but similar to that on Triton.

Another interesting feature on Orcus is the presence of crystalline ice on its surface – which may be an indication of cryovolcanism – and the possible presence of ammonia dissolved in water and/or methane/ethane ices. This would make Orcus quite unique, since ammonia has not been detected on any other TNO or icy satellite of the outer planets (other than Uranus’ moon Miranda).

In 2011, Mike Brown and T.A. Suer detected a satellite in orbit of Orcus, based on images taken by the Hubble Space Telescope on November 13th, 2005. The satellite was given the designation S/2005 (90482) before being renamed Vanth on March 30th, 2005. This name was the result of an opinion poll where Mike Brown asked readers of his weekly column to submit their suggestions.

The name Vanth, after the Etruscan goddess who guided the souls of the dead to the underworld, was eventually chosen from among a large pool of submissions, which Brown then submitted to the IAU. The IAU’s Committee for Small Body Nomenclature assessed it and determined it fit with their naming procedures, and officially approved of it in March of 2010.

Vanth orbits Orcus in a nearly face-on circular orbit at a distance of 9030 ± 89 km. It has an eccentricity of about 0.007 and an orbital period of 9.54 days. In terms of how Orcus acquired it, it is not likely that it was the result of a collision with an object, since Vanth’s spectrum is very different from that of its primary.

Therefore, it is much more likely that Vanth is a captured KBO that Orcus acquired in the course of its history. However, it is also possible that Vanth could have originated as a result of rotational fission of the primordial Orcus, which would have rotated much faster billions of years ago than it does now.

Much like most other KBOs, there is much that we still don’t know about Orcus. There are currently no plans for a mission in the near future. But given the growing interest in the region, it would not be surprising at all if future missions to the outer Solar System were to include a flyby of this world. And as we learn more about Orcus’ size, shape and composition, we are likely to see it added to the list of confirmed dwarf planets.

We have many interesting articles on Dwarf Planets, Kuiper Belt Objects, and the Outer Solar System here at Universe Today. Here is What is a Dwarf Planet? and What is the Kuiper Belt?

And be sure to checkout How Many Planets are in the Solar System?, and this article about all the Bright Objects in the Kuiper Belt.

For more information on Orcus, Vanth, check out the Planetary Society’s page on Orcus and Vanth. To learn more about how they were discovered, consult Mike Brown’s Planets.

Astronomy Cast also has a great interview with Mike Brown from Caltech.